David’s Cisco Networking Blog

Sep 22 2008   6:48AM GMT

Keep it simple with Route Summarization

David Davis David Davis Profile: David Davis

Summarization lumps a series of subnets, or supernets as they are sometimes called, into a single address. This summarization of various subnets have several advantages. It can drastically reduce the size of your routing table and the recalculation of your network because your routing tables are so much smaller.

Unlike EIGRP and OSPF, RIP and IGRP protocols are two examples of automatic summarization because they do not send the subnet mask to the routing table so you can advertise 1 network instead of several networks.  Class C networks are usually subnetted by ‘borrowing’ host address bits and using them for a portion of the network.  You would enable router summarization with the auto-summary command in global configuration mode. An example of continuous subnets would be converting 211.30.192.0/24 through 211.30.195.0/24.

The end result that you would advertise would be 211.30.192.0/22.

Check out the article on IP Routing Protocols for more information.

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